The present invention relates to a unique process for the recovery of substantially radium free calcium sulphate, yttrium and lanthanides in a phosphoric acid preparation process comprising the steps of digesting phosphate rock in the presence of sulphate ions, with an excess of phosphoric acid required for the formation of monocalciumphosphate, separating out the insoluble residue, precipitating calcium sulphate from the remaining solution with sulphuric acid and separating the calcium sulphate from the phosphoric acid solution formed.
One process for the preparation of phosphoric acid and calcium sulphate is disclosed in the U.S. Pat. No. 2,531,977. In the process described therein, phosphate rock is digested with recirculated phosphoric acid product, which contains 0.15-1% free sulphuric acid, subsequently the insoluble residue is separated out, the remaining solution treated with sulphuric acid during which calcium sulphate is precipitated, and the calcium sulphate separated from the phosphoric acid solution formed. The residue separated out is washed with water to remove soluble phosphates. The washing water is then added to the phosphate rock digestion liquor and the remaining solid residue is discharged. While in this known process a fairly pure calcium sulphate is obtained as a byproduct, the lanthanides and yttrium present in the phosphate rock are lost. Moreover, the radium content of the calcium sulphate obtained is still relatively high, which may form an obstacle for various industrial applications, particularly applications in the building industry.
A process for the recovery of practically radium free calcium sulphate in a wet process phosphoric acid preparation is disclosed in the German Auslegeschrift No. 2,218,382. To this end a barium compound is added, in the presence of sulphate ions, to the liquid obtained by digesting phosphate rock with an excess of phosphoric acid, and the radium-containing precipitate is separated off.
One disadvantage of this process is that for the precipitation of the radium a barium compound must be used which has an unfavorable effect on the cost of the process in view of the market price of barium compounds. Moreover, in this process, the valuable lanthanides and yttrium present in the phosphate rock will also be lost.